Field of the Invention
The invention relates to anchoring devices and, in particular, to bone anchors for attaching tissue to bone and a method for using the same.
Description of the Background Art
In the medical and surgical fields, it is sometimes necessary to perform procedures to reattach soft tissue to bone. Soft tissue, such as a tendon or ligament, may rupture, become damaged, or detach from a patient's bone as a result of injury or a medical procedure. Injuries of this type include, torn rotator cuffs, labral tears, bicep tendon tears and quadricep ruptures. Surgical treatment of a torn rotator is designed to reattach the damaged tendon(s) back to the humeral head (ball of the shoulder joint) from which it was torn. Other torn tendons and ligaments require similar procedures.
Bone anchors are one medical implant than can be used to attach soft-tissue to the bone. These anchors are designed to hold the tissue onto the bone at its point of reattachment to allow the tissue to heal and naturally reattach itself to the bone. Typically, the anchor is implanted into a bore or tunnel predrilled into a bone mass and the tendon or ligament is reconnected to the bone with a suture that is attached to the anchor. With advances in arthroscopic surgery, the use of suture anchors has become more popular because of the ease and speed of their use and because of the decreased surgical exposure and morbidity.
Suture anchors provide a stable connection point on the bone for one or more sutures. The sutures themselves tie onto the soft tissue that is being reattached. This combination allows for a firm but flexible bond between the bone and the soft tissue while the natural bond reforms on the bone head.
The implanted suture anchors remain in the bone after the tendon or ligament is fully healed. However, in the event of a false application during the surgical procedure, the anchor must be removable. As a result, it is important that the anchors themselves are both a durable fastener and easily removable. Additionally, anchors should be flush with the bone structure when completely installed to avoid any future irritation.
In prior anchor systems, known in particular from US Patent Publication 2009/0312794 A1, the anchor element is designed such that it has an approximately cylindrical body on whose outer face there are projections that prevent removal of the anchor element after it has been inserted into the bone. These projections can be designed as barb-like elements, for example if the anchor element is driven into the bone, or they can also be designed as an outer thread if the anchor element is turned into the bone in the manner of a screw. The suture is threaded through the transverse bore extending through the body, and the two suture ends are placed in outer longitudinal grooves on the body and guided in the proximal direction. A device called a driver is engaged on the proximal end of the anchor element, and it is usually mounted onto the proximal end of the anchor element. The two suture ends are guided along the driver device and are wound there onto radially projecting stubs for the driving-in procedure.
After the anchor element has been driven into the bone and the driver device has been removed, the two free suture ends are used to secure the detached tissue. To do so, the two suture ends are knotted onto the detached tissue, for example a tendon, lying closely on the bone. The anchor element anchored in the bone, and the bone itself, form the force/abutment points between which the tissue is fixed.
A disadvantage of this operating technique is that the knotting requires considerable experience and dexterity on the part of the operating surgeon. Such knots can come undone, or soft-tissue bridges can form around the knot because the knot is arranged on the outside of the operating site.
Subsequently, so-called knotless anchors were developed, which are known for example from U.S. Pat. No. 7,517,357. This anchor element has a body on whose outer face there are projections that prevent removal of the anchor element inserted in the bone. A transverse bore is arranged in the distal end area of the body and extends through the latter. A suture is threaded transversely through the body. A clamp element is provided which is moveable along the body and is used to clamp the suture. The clamp element is designed as an outer axially moveable sleeve.
In this system, the suture is first threaded through the anchor element. One of the free suture ends is pushed, mostly with the aid of a needle, through the tissue to be fixed, and the pushed-through end is then threaded back in the opposite direction through the transverse bore in the anchor element. The connection between the anchor element and the tissue to be fixed takes the form of a suture loop. The anchor element can now be introduced into the bone, together with the suture after which the free suture ends are pulled so that the protruding loop of the suture, connected to the tissue, is drawn toward the fixing location.
The relative position between the suture and tissue connected to it, and the anchor element is now fixed not by forming a knot, but instead by moving a clamp element through which the suture is fixed or as it were clamped in a defined position on the anchor element. In this way, the loop holding the tissue is also fixed. The protruding free ends can then be cut off, for example, and there is no need to apply a knot.
In the knotless anchor, the clamp element is designed as a sleeve which is mounted on the outside of the body of the anchor element. The sleeve and body are displaceable relative to one another.
In one position of displacement of the sleeve, the suture threaded through the body is freely movable, for example so that the tissue pierced by the suture can be drawn onto the bone and fixed in its position. The sleeve is then moved in order to clamp the suture and fix it in its relative position.
As can be seen in particular from moving from FIG. 4 to FIG. 5 of U.S. Pat. No. 7,517,357, there are several relatively sharp-edged clamp points between which the suture is squeezed. This results in relatively high shearing forces, which means that damage to the suture, and therefore tearing-off of the suture, cannot reliably be ruled out.
In addition, the outer sleeve is a very complicated structural part which, in order to exert a clamping force, has to be slightly spread open by the anchor element. For this purpose, suitable lock-type bridges are needed between the outer face of the body of the anchor element and the inner face of the sleeve, which make release from this locked position difficult or impossible. For this reason, corrective measures, for example during temporary release of the clamping connection, can only be carried out with difficulty, if at all.
It should be borne in mind that the dimensions of such clamp elements are on the order of several centimeters with diameters of several millimeters. Therefore, not only is the production of such parts extremely complex, their handling is also very difficult and, in particular, their stability in respect of the holding or fixing force is extremely problematic.
If a tendon subjected to high loading, for example a tendon from the shoulder area or the knee area, is fixed, it is evident that considerable tensile forces from the tendon act on the assembled structure introduced into the bone and composed of body, clamp element and clamped suture.
If one considers the aforementioned dimensions, it will be evident that the wall thickness of the outer sleeve may at best be in the range of fractions of millimeters, although it is this structural part that is intended to provide the clamping force for holding the suture.
Since the sleeve, because of its construction, covers a certain proportion of the outer face of the body of the anchor element, but this anchor element also serves to hold the whole assembly in the bone via the projections present on its outer face, suitable structural measures have to be taken to ensure that the body of the anchor element as such can in fact be safely anchored in the bone.
Because of the variability of bone strength between people and the numerous locations in which the anchor may be implanted, the anchor may also require expandability to lock it into softer bone tissue. Depending on the application, the whole shaft may need to expand for a complete lock, whereas in other instances only the distal end needs to expand.
In addition, torque from a driver must be carefully applied to ensure that the sleeve and inner screw do not strip and lock irreversibly or over-expand and break. The head of the inner screw, the threads of the inner screw, and the thin walls of the sleeve are all susceptible to damage from over application of torque. Furthermore, the suture and surrounding bone tissue can be damaged if torque is applied after the inner screw has been fully inserted.